Along with recent technological development, more highly integrated and higher density semiconductors have been developed. This is associated with a tangible increase in wiring delay resulting from a progress in micronization of wiring width and wiring pitch.
The wiring delay is proportional to the product of wiring resistance and capacitance between wires and wiring resistance tends to be increased along with the aforementioned micronization of wiring width. Therefore, an increase in capacitance between wires leads to a remarkable increase in wiring delay, bringing about a deterioration in circuit operation speed resultantly. Therefore, in order to raise the circuit operation speed of a semiconductor device, adoption of low-resistance wiring materials and layer insulting films having a low dielectric constant and establishment of process technologies for producing a semiconductor device by using these materials and films are being sought.
As current wiring materials of a semiconductor integrated circuit, aluminum or aluminum alloys are primarily used. In integrated circuits that are expected to be operated at a higher speed, copper, which is a metal having lower resistance than aluminum, is adopted as wiring material to thereby reduce wiring resistance.
In the meantime, in order to reduce the capacitance between wires, it is necessary to drop the dielectric constant of an insulating film enclosing a wiring member. It is therefore effective to adopt insulating materials having a lower specific inductive capacity than inorganic type materials such as SiO2 (specific inductive capacity is 4), SiON (specific inductive capacity is 4 to 6) and Si3N4 (specific inductive capacity is 7), which have been utilized so far. Specifically, fluorine-added SiO2 (SiOF), organic type insulating film materials containing a carbon atom or the like are currently utilized as materials having a low dielectric constant.
Although the fluorine-added SiO2 can reduce specific inductive capacity when fluorine in the film is concentrated, it raises a problem that moisture absorption increases. Further, there is another problem that hydrogen fluoride produced by a reaction between moisture and hydrogen corrodes wiring materials and a rise in specific inductive capacity due to desorption of fluorine. Accordingly, the specific inductive capacity obtained substantially by fluorine-added SiO2 is about 3.3 and it is difficult to attain a specific inductive capacity of 3 or less.
On the other hand, examples of the organic type insulating film material containing a carbon atom include organic SOG (spin on glass), polyimides, carbon-added SiO2 (hereinafter referred to as SiOCH) using organic silane gas, or insulating films (hereinafter referred to as a porous film) obtained by making these materials porous. Among these materials, especially, the porous insulating film can attain a specific inductive capacity of 2.8 or less because it contains pores therein and is one of the materials expected to act as interlayer insulating films of ULSI multilayer copper wiring next to the 90 nm generation.
Examples of one of the methods of forming porous insulating films include a spin coating method. In the spin coating method, for example, a solution prepared by mixing a raw material monomer and a pore forming agent is applied by spin coating (the solution is dripped on the surface of a rotating silicon substrate) to form a film on the silicon substrate. Then, the silicon substrate is heated to 350° C. to 450° C. to cause a thermal polymerization reaction and the thermal decomposition dissociation reaction of the pore forming agent in the film, thereby forming a porous insulating film containing pores in the film. This spin coating method is a method that is widely used as the method of forming an insulating film. However, this method has the following problem.
In the spin coating method, a monomer solution prepared by dissolving monomers in an organic solvent is applied by spin coating and uniformed. At this time, about 90% of the solution is splashed off the substrate and becomes a waste solution. Therefore, the utility effect of a raw material is low and the production cost is therefore increased.
Also, in the spin coating method, a heating step for causing a polymerization reaction and a curing reaction of the monomers is required. In this heating step, generally, heat treatment is carried out for several tens minutes to several hours in a furnace kept at a temperature of about 300 to 450° C. and therefore, the throughput of the whole process for producing a device is dropped. Also, when oxygen molecules are present in the air during heating, these oxygen molecules react with the monomers and there is the case where an intended film structure is not obtained. It is therefore necessary to substitute the whole gas of the baking furnace with inert gas to remove oxygen molecules in the air during heating, which is a hindrance to cost reduction.
Moreover, a coating and heating step is carried out in each layer when multilayer wiring is formed and therefore the lowermost wiring layer repeatedly receives heat stress at high temperatures for a long time. This is a cause of a reduction in reliability, especially, of the deterioration of fined copper wiring.
Also, the porous insulating film obtained by a spin coating method has pores continuing to the surface of the film because of its formation principle. In more detail, in the spin coating method, regions where the pore forming agent in the film are thermally decomposed and dissociated are obtained as pores. To achieve this, it needs gas vents through which the heat-decomposed and dissociated gas is emitted from the inside of the film to the outside (film surface). Because the pores in the film are always connected to the gas vents, the pores are continuous pores continuing to the surface of the film. Specifically, in the case of the spin coating method, since the polymerizing and growing step for forming an insulating film by utilizing a thermal reaction is independent of the step of forming pores, the porous insulating film has a structure provided with continuous pores leading to the surface of the film consequently.
However, these continuous pores leading to the surface of the film act as passages through which moisture in the outside air and etching gas or cleaning water used for semiconductor processing penetrate and diffuse. As a result, the characteristics of the porous insulating film changes with time, resulting in an unstable porous film.
As another method of obtaining a porous insulating film, a plasma CVD method forming an amorphous insulating film is taken. In this plasma CVD method, a raw material gas is dissociated and activated in plasma to form an amorphous insulating film. This method has the advantage superior to the coating method in the point of utility efficiency of a raw material and the raw material is made into a thin film more easily than the spin coating method. Also, this method has the advantage that unlike the coating method, it needs no curing step with heating.
However, in the above plasma CVD method, the starting raw material gas is dissociated into an atomic level in plasma and on principle, it is not that a structure reflecting the molecular skeleton of the starting material is grown. For example, when a straight-chain organic silica molecule is dissociated in plasma, some of activated silica molecules are bonded in the plasma gas before they reach a silicon substrate and are accumulated like falling snow to form a bulky porous film on a substrate.
In this case, it is necessary to prolong the time for the activated atoms to stay in plasma in order to promote the linking reaction of the activated atoms in the plasma. Specifically, it is necessary to keep the substrate sufficiently apart from the plasma. However, it is difficult to control the process of forming a film of the activated atom in the plasma and it is difficult to control the structure of a growing porous insulating film. Namely, it is basically difficult to control the size and filling density of pores in the film for obtaining a low dielectric constant. Also, because this method needs large plasma power to dissociate the starting material, it gives rise to the problem that a semiconductor device receives a large damage. Moreover, when the pore diameter is increased to about 3 nm or more, moisture, process gas, chemical solutions and the like easily enter into the insulating film and the strength of the film lowers, with the result that this causes a deterioration in the reliability of the insulating film and lower adaptability to a process of manufacturing a semiconductor device.
Also, when the porous insulating film is utilized as a layer insulating film, it must have high adhesiveness to other semiconductor materials that are in contact therewith. In order to develop the interlayer insulating film having a lower dielectric constant, it is effective to decrease the ratio of polar elements (for example, oxygen and silicon) contained in the film, specifically, to increase the ratio of organic groups in the film. However, if the ratio of polar elements is decreased, the surface density of these polar elements is decreased on the interface between the film and other semiconductor materials and also the film has such a structure that its composition abruptly changes at the contact interface, with the result that it is difficult to maintain high adhesiveness.
Japanese Patent Application Laid-Open No. 2001-230244 named “METHOD OF FORMING INSULATING FILM AND MULTILAYER WIRING” (Patent document 1) aims to lower the specific inductive capacity of an insulating film. The invention efficiently forms a film of benzocyclobutene having high heat resistance by growing plasma polymerization divinylsiloxane-bis-benzocyclobutene on the surface of a semiconductor substrate.